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Questions being asked in the interview :
1.What is a differential amplifier? Also, explain CMRR.
Ans.The amplifier, which is used to amplify the voltage difference between two input-lines neither of which is grounded, is called differential amplifier. This reduces the amount of noise injected into the amplifier, because any noise appearing simultaneously on both the input- terminals as the amplifying circuitry rejects it being a common mode signal.
CMRR: It can be defined as the ratio of differential voltage-gain to common made voltage gain. If a differential amplifier is perfect, CMRR would be infinite because in that case common mode voltage gain would be zero.
2. Why star delta starter is preferred with induction motor?
Ans.Star delta starter is preferred with induction motor due to following reasons:
-Starting current is reduced 3-4 times of the direct current due to which voltage drops and hence it causes less losses.
-Star delta starter circuit comes in circuit first during starting of motor, which reduces voltage 3 times, that is why current also reduces up to 3 times and hence less motor burning is caused.
3.State the difference between generator and alternator
Ans.Generator and alternator are two devices, which converts mechanical energy into electrical energy. Both have the same principle of electromagnetic induction, the only difference is that their construction. Generator persists stationary magnetic field and rotating conductor which rolls on the armature with slip rings and brushes riding against each other, hence it converts the induced emf into dc current for external load whereas an alternator has a stationary armature and rotating magnetic field for high voltages but for low voltage output rotating armature and stationary magnetic field is used.
4.Why AC systems are preferred over DC systems?
Ans.Due to following reasons, AC systems are preferred over DC systems:
a. It is easy to maintain and change the voltage of AC electricity for transmission and distribution.
b. Plant cost for AC transmission (circuit breakers, transformers etc) is much lower than the equivalent DC transmission
c. From power stations, AC is produced so it is better to use AC then DC instead of converting it.
d. When a large fault occurs in a network, it is easier to interrupt in an AC system, as the sine wave current will naturally tend to zero at some point making the current easier to interrupt.
5.What are the various kind of cables used for transmission?
Ans.Cables, which are used for transmitting power, can be categorized in three forms:
- Low-tension cables, which can transmit voltage upto 1000 volts.
- High-tension cables can transmit voltage upto 23000 volts.
- Super tension cables can transmit voltage 66 kV to 132 kV.
6.What are the different methods for the starting of a synchronous motor.
Ans.Starting methods: Synchronous motor can be started by the following two methods:
- By means of an auxiliary motor: The rotor of a synchronous motor is rotated by auxiliary motor. Then rotor poles are excited due to which the rotor field is locked with the stator-revolving field and continuous rotation is obtained.
- By providing damper winding: Here, bar conductors are embedded in the outer periphery of the rotor poles and are short- circuited with the short-circuiting rings at both sides. The machine is started as a squirrel cage induction motor first. When it picks up speed, excitation is given to the rotor and the rotor starts rotating continuously as the rotor field is locked with stator revolving field.
(Written on: 21.10.2015)